1. Field of Invention
This invention relates to a method of controlling the consistency of massecuite in an automatic boiling apparatus in a pan.
2. Description of the Prior Art
FIG. 1 depicts a conventional vacuum boiling apparatus, comprising a parallel side pan 1 having a calandria type heating area 2. The solution F (e.g. syrup used to prepare sugar), to be boiled, is supplied into the bottom of pan 1 through a solution control valve 3, Heating steam S is supplied to heating area 2 through a control valve 4 to heat and concentrate the solution by vaporization. The solution continues to be supplied until a concentration enabling crystallization is reached. Then, a seed is added from a feeder 5 through a valve 6 to form appropriate nuclear grains.
While the interior of pan 1 is observed, such as through windows 10, water W or solution F is supplied to avoid bonding of the nuclear grains and formation of undesirable nuclear grains (false grains), so that the concentration of the solution and the growing of crystals may be continued. If crystals grow to a certain extent, false grains are less likely to form, since the crystals occupy a certain volume in a unit volume of messecuite (defined as a mixture of solution and crystals) and are located relatively close to one another. The solution is further concentrated to facilitate growth of crystals. Solution F is added to increase its volume in the pan 1, to a certain level. When a predetermined crystal size has been obtained, the massecuite 7 is discharged through a discharge valve 8.
The massecuite is separated by a centrifugal separator into the crystals and the solution. The solution is then recycled for boiling. In order to control the concentration of the massecuite appropriately during boiling, it is possible to supply pan 1 with water W or solution F through a water control valve 9 or solution control valve 3. Of course if necessary or desired suitable amounts of both may be concurrently supplied. It is possible to observe the interior of pan 1 through peep windows 10. Steam is drawn out from pan 1 into a condenser 11 by a vacuum pump 13 which is connected to condenser 11 through a valve 12. Condenser 11 is cooled by cooling water W which is supplied through valve 14.
Various methods have been proposed for controlling the pan, but the usual method used now is to use an intermittent boiling method which increases the consistency of massecuite in accordance with a control method comprising a specific program of steps and thereby attains stability of operation. One such method is disclosed in Japanese Laid Open Pat. No. 41248/1977.
Returning to FIG. 1, a signal e.sub.m, indicating the consistency of massecuite, is transmitted from a consistency meter 15, such as a rheometer, to control portion 161 of a sequence control system 16. The control system 16 also includes a program setting system 162 for feeding a set value e.sub.s of of a particular consistency desired to the control portion 161, and a valve actuator 163 for opening or closing solution control valve 3 and/or water control valve 9 in accordance with the output of the control portion 161.
A level gauge for the determining of the level of the massecuite 7 in pan 1, a pressure control device for maintaining an appropriate vacuum degree in pan 1, etc, are also provided, although not shown in FIG 1.
A conventional method of the program control of the boiling operation, such as of the system of FIG. 1, is depicted in FIGS. 7A, 7B. FIG. 7A shows changes in the measured value e.sub.m and set value e.sub.s of consistency in a specific area of the crystal growing process in which the solution is boiled. FIG. 7B depicts the operation of the solution control valve 3 at different times.
At time t.sub.1, when measured value e.sub.m has increased to the level m.sub.1 of set value e.sub.s, solution F is supplied to pan 1 to control, such as by loosening, the consistency of massacuite. The next level m.sub.2 of the set value e.sub.s is higher than the level m.sub.1 by .DELTA.m. When the value e.sub.m has increased to level m.sub.2, solution F is supplied again at time t.sub.2. The same is repeated at time t.sub.3, time t.sub.4, etc. A broken line C, which is obtained by connecting the peak values of e.sub.m, defines the ideal limit curve for the control of consistency. If the consistency of the massecuite is controlled in accordance with curve C, it is possible to complete a batch of operation in a minimum amount of time, while maintaining the high quality of crystals.
The ideal curve C can, however, be maintained only when various parameters, such as the amount of steam in the pan, its vacuum degree and purity of solution, are maintained at suitable levels. It is difficult to maintain any such ideal pattern of control if one or more of the parameters change, for example, if the amount of steam S in the pan or its vacuum degree has substantially changed.
For example, if the amount of steam has been reduced abnormally after time t.sub.4, a long time is required for value e.sub.m of consistency to reach the set value e.sub.s at m.sub.5. Thus, if the same pattern of control is continued, the value of consistency changes to e.sub.m ', as shown in FIG. 7A. A curve C', which is obtained by connecting the peak values of e.sub.m ', has a lower gradient than curve C and largely deviates therefrom. If boiling is continued under these circumstances, a prolonged time is required for a batch of operation to be completed, and it is difficult to obtain crystals of good quality, since false grains are likely to form.
In another example, as shown in FIG. 7A, an abnormal increase in the amount of steam after time t.sub.4 gives rise to a phenomenon contrary to what has been above described. The value of consistency changes to e.sub.m ". A curve C", which is obtained by joining the peak values of e.sub.m ", has a higher gradient than curve C and largely deviates therefrom. A batch of operation is completed abnormally rapidly resulting in the production of defective products containing a large amount of false grains.